1. Field of the Invention
This invention relates to an image forming apparatus, such as a copier, a printer, a facsimile machine, a printer, etc.
2. Description of the Related Art
Conventionally, image forming apparatuses are known that reduce density irregularity of a toner image formed on an image bearer.
For example, Japanese Patent Publication No. 62-145266 (JP-S62-145266-A) discloses a technology in which an image recorder (i.e., an image forming apparatus) scans a modulated laser beam onto a photoconductive drum (i.e., an image bearer) to record a latent image thereon and then applies an electro-photographic process thereto to execute development and transfer processes to obtain an output of an image. Prior to such an image output, the recorder records a solid black image on the photoconductive drum and reads the solid black image to obtain information that is read and stored in a memory to correct image density at each recording position based on the information read and stored.
Japanese Patent Publication No. 09-62042 (JA-H09-62042-A) discloses an image forming apparatus that reduces density irregularity periodically occurring on an image by controlling at least one of several formation conditions including a charging voltage, a light exposure amount, a developer voltage, or a transfer voltage based on data on periodic fluctuations of image density or a charge potential on an image bearer, each of which has been previously stored. Such periodic fluctuation data used in controlling an image formation condition is measured beforehand based on a single type of image data (e.g. a solid image) in the image forming apparatus.
Japanese Patent No. 3825184 (JP-3825184-B) discloses an image forming apparatus that detects a rotation cycle of a developing roller with a developing roller cycle detector and detects an amount of irregularity of toner density in a pattern formed on an image bearer. The image forming apparatus then controls a developing bias by matching a phase of an output signal from the above-described density irregularity amount detector with that of an output signal from the developing roller rotation cycle detector. Accordingly, the density irregularity of the solid image can be corrected by varying the development potential during the above-described developing bias control process executed in the image forming apparatus.
Japanese Patent Publication No. 2006-106556 (JP-2006-106556-A) also discloses an image forming apparatus that forms a test image on an image bearer or a transfer medium, and detects a frequency of image density irregularity periodically occurring thereon. The image forming apparatus then identifies a source of the image density irregularity based on the detected frequency to control an operation of the source thereof to reduce image density irregularity.
However, image density irregularity cannot be reduced completely across multiple different types of images of varying image densities (for example, a solid image and a halftone image) by the above-described approaches. For example, with the image forming apparatus of JP-62-145266-A, density irregularity of a halftone image cannot be reduced and is worse when it is formed, although density irregularity of a solid image can be reduced because those are corrected based on information read from a solid black image. Further, periodic variation data, such as image density, etc., used in controlling an image formation condition is measured based on image data of a single type image (for example, a solid image) in the image forming apparatus of JP-09-62042-A. However, a profile of irregularity of image density is sometimes slightly different depending on the level of image density (e.g. a level of a solid image portion or that of a halftone image portion). Further, in the image forming apparatus of JP-3825184-B, since a development potential is changed by controlling a developing bias, thereby changing a background potential at the same time, a halftone image is affected by fluctuation of the background potential and fluctuates unexpectedly. Therefore, it works well to correct the density of the solid image but not that of the half-tone image. Further, in the image forming apparatus of JP-2006-106556-A, when a frequency of periodic density irregularity in each of solid and halftone images is detected, similar frequency property is probably detected even if a source of generation of density irregularity is different in each of the images. In such a situation, the source of generation of density irregularity of each of the images may not be accurately identified, so that the density irregularity cannot be reduced appropriately.